Freshman in high school? You're doing the right thing!

Reading plays and watching theatre is what you should be doing right now. Go see local plays! Read their descriptions, pop into anything that looks interesting. Big theatre or small theatre. See both! Since you're a student, you can show up last minute to tons of theatres, and they'll let you in super cheap (this works better in bigger cities). It's called student rush. Call them and make sure they do it beforehand.

And reading Shakespeare is fantastic and few people your age are doing it. Read some recent plays too! You'll probably enjoy them. Anything that's won a Pulitzer is a great bet. Who's Afraid of Virginia Woolf, Angels in America, plenty of Tennessee Williams, Rosencrantz and Guildenstern are Dead, plenty of Sam Shepard, and other modern classics are important. But read some contemporary work! Try reading something written by Sarah Ruhl or Annie Baker, like The Vibrator Play, or The Flick.

If you really want to branch out and do more than this, read A Challenge for the Actor by Uta Hagen. That's a great place to start. Watch some videos of actors you like discussing the craft on Youtube. Try to apply what you learn to a monologue, maybe.

Don't rush, there's no need to. Soon enough, see if local theatres ever need to cast teenagers. I'm sure some do. Audition for those roles when they pop up. In a few years, research colleges, find an audition coach. I decided I wanted to act in my junior year, and still got into a top conservatory. I think it was honestly just because I loved theatre and was well read, so I poured my heart into my audition and practiced it a ton.

So be doing that! Develop a love for theatre, and read a lot!